The invention relates to a process for the production of a thin dielectric capacitive hygrometer, as well as to a hygrometer obtained by this process.
For measuring the humidity of the air, it is known to use the variations of the dielectric constant of a thin layer material constituting the dielectric of a capacitor as a function of the quantity of water absorbed.
A certain number of devices functioning on the basis of this principle already exist and have been described in the literature. Reference is made in this connection to U.S. Pat. Nos. 3,168,829 (Nelson) and 3,350,941 (Misevich et al), British Pat. No. 1,297,014, the Article by K. W. Misevich in IEEE Trans. and Ind. Electronics and Control Instrumentation, Vol. IECI 16, No. 1, July 1969, Finnish Pat. No. 2,831,72 of the Vaisala Company, French Patent No. 73 36153 of the same company and corresponding to the aforementioned Finnish Patent, French Pat. No. 76 01904 of the Commissariat a l'Energie Atomique (Barraud, Messier, Rosilio), the Article by E. Salasma and P. Kestamo of the Vaisala Company (3rd Symposium on Meteorological Observations, February 1973, Washington), the Article by P. E. THOMA, J. O. COLLA and R. STEWART in IEEE Trans. on Comp Hybrids and Manufact. Technology, Vol. CHAT, No. 3, Sept. 1979 and finally the 1974 U.S. Pat. No. 3,802,268 of P. E. THOMA.
These different hygrometers or humidity measuring devices can be placed in two categories. The first category (Misevich, Nelson, Thoma) is characterized by the fact that the dielectric material is formed by a polymer sheet serving both as the sensitive element and as the mechanical support for the hygrometer. This leads to a certain number of disadvantages for this type of hygrometer, i.e. limited mechanical strength, deformation of the polymer leading to a significant hysteresis effect and a certain temperature coefficient.
The hygrometers of the second type are formed from a polymer layer (Vaisala) or other water-sensitive molecules (Barraud et al) deposited on a metallized solid substrate forming the first electrode of a capacitor. Thus, the mechanical support of the hygrometer, generally made from glass or some other moisture-inert material, is separate from the water-sensitive layer.
This arrangement makes it possible to overcome the defects of the first type (mechanical strength, hysteresis, slow response, temperature coefficient), but the second electrode of the capacitor must be very thin to enable it to be permeable to water. This makes it necessary to take certain precautions during manufacture and leads to a certain lack of strength on the part of the hygrometer. In addition, the thinness of the electrode makes it sensitive to pollution, which reduces the service life of the device. Finally, it is a difficult operation to deposit the polymer film, because the latter must be free from defects or holes in order to prevent any danger of short-circuiting between the electrodes. However, to obviate this danger, it is necessary to place a thin insulating layer, which is insensitive to water between the first electrode and the polymer layer. This prevents short-circuits in the case of any perforation of the dielectric without modifying the sensitivity of the hygrometer, because the insulating layer has a significant capacitance.